In any environmental management system (EMS), there is a requirement to determine organization’s environmental aspects that are associated with its activities, taking into consideration life cycle perspective. Further the EMS requires that from all the aspects, significant environmental aspects should be identified. Significant environmental aspects are those which cause or can cause significant impact on the environment.
Identification of significant environmental aspects is done by defining and applying criteria of significance. There are a few ways to do it. One such criteria is known by acronym SSPD (Scale, Severity, Probability & Duration). It was first developed by US EPA many years ago. Original criteria is as below.
| Parameter | Rating Categories | ||||
| 1 | 2 | 3 | 4 | 5 | |
| SCALE | insignificant volume/ quantity | low volume/ quantity | medium volume/ quantity but sporadic | medium volume/ quantity but ongoing | high volume/ quantity |
| SEVERITY | minimal impact | moderate impact but localized and readily containable | moderate impact over multiple locations | significant impact and/or regional | extreme impact and/or potential for global impact |
| PROBABILITY | very unlikely under any operating condition | occurs during abnormal/emergency conditions/probability anticipated and managed | occurs during small-medium new projects or routine maintenance activities | occurs during major new projects or major maintenance activities | occurring during normal operating conditions and artifact of operations |
| DURATION | spike situation extremely short-term duration within one day | less than one month | one-six months | less than one year | long-term duration greater than one year or continuous |
The above criteria of significance for environmental aspects is modified by an organization to suit its own operations. Basic logic is not changed. It is shown below.
| Parameter | Rating Categories | ||||
| 1 | 2 | 3 | 4 | 5 | |
| SCALE (Rate of release of pollutant) | Less than 1 litre/kg per hour, or Pollutant within radius of 1 meter | Less than 20 litre/kg per hour, or Pollutant within radius of 10 meters | Less than 50 litre/kg per hour but sporadic, or Pollutant within radius of 50 meters | Less than 50 litre/kg per hour but ongoing, or Pollutant within radius of 50 meters | Release rate > 50 litres/ kg per hour. Or Pollutant spread beyond boundary wall |
| SEVERITY | Involves non-toxic material, or Potentially minimal impact | Involves non-toxic material, or Potential for moderate impact and localized | Involves toxic* material, or Moderate impact | Involves highly toxic* material, or Significant impact | Involves extremely toxic* material, or Extreme impact on environment |
| PROBABILITY | Once in a year or less, or Very unlikely under any operating condition | Once in few months, or Occurs during abnormal/emergency conditions | Once in a month, or Occurs during projects execution or major maintenance activities | Once in week | One or more times in a day, or Occurring during normal operating conditions and artifact of operations |
| DURATION | Spike situation – short term duration | Duration of release less than 1 hr | Duration of release/s between 1-8 hrs | Duration of release between 8-24 hrs | More than a day |
* Criteria of toxicity given in MSIHC Rules 1989
Magnitude of risk (i.e., damage) is = one time impact X frequency of such impacts taking place
Environmental Risk = Environmental impact X Frequency (or probability) of its happening
Total pollutant released = Rate of release of material X duration of release
Environmental damage = total pollutant released X toxicity (or severity)
= Rate of release of material X Duration of release X Severity
= Scale X Duration X Severity
Hence,
Environmental risk = (Scale X Duration X Severity) X Probability
= SSPD
An example to assess – assume we are assessing environmental aspect of chlorine gas in a chlorination process. Whenever chlorine cylinder is changed there is venting of gas present in piping, before joint is broken and remade with new cylinder in position. Take venting rate as 42 litres/ hr. Duration during a day cumulatively is 10 minutes. Chlorine is Highly toxic as per MSIHC Rules 1989. The evaluation will then be:
Scale – 3
Severity – 4
Probability – 5
Duration – 1
The environmental risk rating will be = SXSXPXD
= 3X4X5X1 =60
If the limit criteria of significance (which is decided by the organization) is 50 (say). Then this is significant environmental aspect.